Fast neutron nuclear reactors cooled by a liquid metal such as sodium contain a core formed by assemblies with an elongated prismatic shape immersed in the liquid sodium contained in the reactor vessel.
It may become necessary to remove the fuel assemblies from the reactor core inside the vessel, for example in order to replace spent or defective assemblies with new assemblies. The assemblies coming out of the reactor vessel are in an irradiated state and produce radioactive radiation. Moreover, these assemblies have residual activity and are the seat of a release of heat produced by the nuclear reactions in the irradiated fuel. Finally, these assemblies retain a certain amount of liquid sodium at the time when they emerge from the vessel.
The fuel assemblies coming out of the nuclear reactor vessel must therefore be disposed in containers ensuring the biological protection of the medium to which the assemblies are moved or in which they are temporarily stored. It is also necessary to cool the assembly while it is being transferred.
In order to carry out the transfer and temporary storage of fuel assemblies from fast neutron nuclear reactors, it is normal to use casks consisting of a solid body made of a material absorbing nuclear radiation in which a housing is provided for a fuel assembly, either in a bare state or embedded in a casing filled with sodium to cool it. These casks are fitted with valves at their lower part.
The assembly housing, in the shape of a central channel inside the cask body, emerges, through an opening which can be tightly closed, e.g., by a valve, into one of the ends of the cask body, so as to allow the assembly to be introduced into the cask and extracted from it. These operations are carried out after the system consisting of the transfer cask and its valve has been positioned so that it abuts onto a fixed structure, e.g., a structure attached to the closure slab of the reactor vessel, so that the central channel of the cask lies along the axial extension of a well passing through the fixed structure, e.g., of a vertical well passing through the slab.
A sealing device consisting of at least two concentric O-rings is interposed between the lower abutment surface of the valve support fitted on the transport cask and the corresponding abutment surface of the fixed structure. The device for tightly closing the opening in the central channel of the cask emerging on to the abutment surface of the valve support and a second device for tightly closing the upper end of the vertical well of the fixed structure emerging on to the abutment structure of the fixed structure are disposed inside the contact zone of the inner joint of the sealing device, in aligned axial positions.
A device for opening and tightly closing the communication passage between the central channel of the cask and the vertical well has been described in applicant's French Patent Application No. 90-08116, filed on Jun. 27, 1990. When the transport cask and its valve are being placed on the fixed abutment structure, it is necessary to check that this emplacement has been carried out correctly before opening the cask valve and the well valve putting the central housing of the cask into communication with the well passing through the slab. In fact, the well passing through the slab which communicates with the internal volume of the reactor and/or the housing of the cask which is capable of holding an irradiated fuel assembly contain radioactive gases which must be prevented from escaping to atmosphere when the valves are opened.
When it is being placed on the abutment surface, the valve of the very heavy transport cask produces a certain amount of compression of the joints, which enables a tight seal to be obtained as long as the compression of the joints is produced uniformly around their entire periphery. In cases where the cask is presented to the abutment surface in a position slightly inclined to the vertical, the compression of the joints carried by the valve is nevertheless correctly produced since the valve is decoupled from the cask structure by a flexible bellows system.
In order to monitor the emplacement of a transportable element such as a cask and the sealing between the abutment surface of the cask valve and the abutment surface of the fixed structure upon which the cask comes to bear, it is known to use monitoring devices providing for the introduction of a pressurized gas or the reduction in pressure with respect to the atmosphere of the space lying between the joints and bounded by the abutment surfaces of the valve and the fixed structure. The monitoring itself is carried out by using a manometric device to measure the rate of leakage of the pressurized gas in the direction of the atmosphere or of atmospheric air in the direction of the space between the joints.
Such a measurement takes a relatively long time, inasmuch as it requires prior pressurizing or evacuation of the space between the joints and continuous measurement of the pressure in this space over a period of time which may be lengthy. This implies the introduction of additional equipment and hence increased risk of breakdown.
As a result, there is a waiting time in the fuel assembly handling cycle, which correspondingly slows down the loading and unloading operations of the nuclear reactor. In fact, the loading and unloading of the nuclear reactor core necessitates many operations involving transfer of fuel assemblies, so that it is extremely important to reduce the operational time required for the transfer of an assembly as much as possible, with the aim of limiting the period during which the reactor is shut down.
Moreover, the prior art monitoring devices are relatively complex and have to include additional elements such as a reserve supply of pressurized gas or a vacuum pump.